Conventionally, a pressure sensor, an acceleration sensor, an angular velocity sensor, and the like manufactured by micro-electro-mechanical systems (MEMS) technology to which a semiconductor process is applied detect a displacement of a movable element so as to detect a physical quantity (pressure, acceleration, angular velocity, and the like). In general, a change in electrostatic coupling capacitance formed between the movable element and a fixed surface is measured, thereby measuring a distance between the movable element and the fixed surface. Based on a displacement amount of the movable element thus obtained, the physical quantity to be measured can be detected.
On the other hand, Japanese Patent Application Laid-Open Nos. H04-25764 and H08-50022 each propose a sensor for detecting the displacement of the movable element by use of a field-effect transistor (FET) in which a gate of a metal-oxide-semiconductor field-effect transistor (MOSFET) has a suspended-gate structure (hereinafter, referred to as “SG-FET”). FIGS. 10A and 10B are a plan diagram and a cross-sectional diagram, respectively, of the sensor disclosed in Japanese Patent Application Laid-Open No. H04-25764. FIG. 11 is a cross-sectional diagram of the sensor disclosed in Japanese Patent Application Laid-Open No. H08-50022. The SG-FET includes a substrate having a source region and a drain region of the transistor formed thereon, and a movable element formed with a predetermined spacing from a surface of the substrate and having a certain potential. In this case, when the movable element is displaced according to the physical quantity to be detected, the amount of current that flows into a channel between the source region and the drain region is changed correspondingly, thereby measuring an amount of change to detect the displacement of the movable element. In this method, the displacement of the movable element can be directly detected, with the result that degradation of a detection signal due to a wiring extending to a first-stage amplifier circuit can be prevented, and an effect of superimposed noise can be reduced. Further, a small displacement of the movable element can be detected with high sensitivity.
Japanese Patent Application Laid-Open No. H04-25764 discloses such a structure, illustrated in FIGS. 10A and 10B herein, that can be obtained by a method of forming the sensor in which a substrate 1 having a movable element (cantilever 2) formed thereon, and a substrate 7 having a source region (source electrode 8) and a drain region (drain electrode 9) formed thereon are bonded together. (Element 3 is a gate electrode, 4, 5 and 6 denote wiring, 10 is a groove, and 11 denotes the package.) Japanese Patent Application Laid-Open No. H08-50022 discloses a structure, illustrated in FIG. 11 herein, in which a movable portion 13 includes a movable element 16 that is held above a substrate 12, which has a source region 20 and a drain region 21 (connected by channel 28, which is covered by insulated film 22) formed thereon, in a state where the movable element 16 is suspended above the substrate 12. Connections S, D and G for the source, drain and gate are shown, and an electrode 26 is provided on the back of the substrate.